Multi-speed power transmissions, particularly those using planetary gear arrangements, require a hydraulic system to provide controlled engagement and disengagement, on a desired schedule, of the clutches and brakes or torque transmitting mechanisms that operate to establish the ratios within the planetary gear arrangement.
These control systems have evolved from substantially pure hydraulic control systems, wherein all of the control signals are produced by hydraulic devices, to electro-hydraulic control systems, wherein a number of the control signals are produced by an electronic control unit. The electronic control unit emits electrical control signals to solenoid valves, which then issue controlled hydraulic signals to the various operating valves within the transmission control.
With many of the early pure hydraulic and first generation electro-hydraulic control systems, the power transmission utilized a number of freewheel or one-way devices which smooth the shifting or ratio interchange of the transmission during both upshifting and downshifting of the transmission. This relieves the hydraulic control system from providing for the control of overlap between the torque transmitting mechanism that was coming on and the torque transmitting mechanism that was going off. If this overlap is excessive, the driver feels a shudder in the drivetrain, and if the overlap is too little, the driver experiences engine flare or a sense of coasting. The freewheel device prevents this feeling by quickly engaging when the torque imposed thereon is reversed from a freewheeling state to a transmitting state.
The advent of electro-hydraulic devices gave rise to what is known as clutch-to-clutch shift arrangements to reduce the complexity of the transmission and the control. These electro-hydraulic control mechanisms are generally perceived to reduce cost and reduce the space required for the control mechanism.
In addition, with the advent of more sophisticated control mechanisms, the power transmissions have advanced from two-speed or three-speed transmissions to five-speed and six-speed transmissions. In at least one presently available six-speed transmission, just five friction devices are employed to provide six forward speeds, neutral condition, and a reverse speed. Such a gear arrangement is shown in U.S. Pat. No. 4,070,927 issued to Polak on Jan. 31, 1978. The use of the planetary gearset shown in the Polak patent has given rise to a number of electro-hydraulic control mechanisms, such as that shown in U.S. Pat. No. 5,601,506, issued to Long et al. on Feb. 11, 1997.
Additionally, an electrically variable hybrid transmission has been proposed to improve fuel economy and reduce exhaust emissions. The electrically variable hybrid transmission splits mechanical power between an input shaft and an output shaft into a mechanical power path and an electrical power path by means of differential gearing. The mechanical power path may include clutches and additional gears. The electrical power path may employ two electrical power units, or motor/generator assemblies, each of which may operate as a motor or a generator. With an electrical storage system, such as a battery, the electrically variable hybrid transmission can be incorporated into a propulsion system for a hybrid electric vehicle. The operation of such an electrically variable hybrid transmission is described in the published U.S. Patent Application No. 2003/0078126 by Holmes et al. and filed on Oct. 18, 2001.
The hybrid propulsion system uses an electrical power source as well as an engine power source. The electrical power source is connected with the motor/generator units through an electronic control unit, which distributes the electrical power as required. The electronic control unit also has connections with the engine and vehicle to determine the operating characteristics, or operating demand, so that the motor/generator assemblies are operated properly as either a motor or a generator. When operating as a generator, the motor/generator assembly accepts power from either the vehicle or the engine and stores power in the battery, or provides that power to operate another electrical device or another motor/generator assembly.
Two other electro-hydraulic control systems shown in U.S. Pat. Nos. 4,827,806 and 5,616,093, both issued to the assignee of this application, provide drive-home capabilities within the transmission in the event that the electronic system undergoes a malfunction or discontinuance of operation. The drive-home feature of a power transmission is an important factor in that it permits the vehicle operator to return home with the vehicle so that the proper repairs can be undertaken at a repair station rather than in the field where the vehicle underwent the malfunction.